Ecological assembly rules in plant communities—approaches,patterns and prospects

Understanding how communities of living organisms assemble has been a central question in ecology since the early days of the discipline. Disentangling the different processes involved in community assembly is not only interesting in itself but also crucial for an understanding of how communities will behave under future environmental scenarios. The traditional concept of assembly rules reflects the notion that species do not co‐occur randomly but are restricted in their co‐occurrence by interspecific competition. This concept can be redefined in a more general framework where the co‐occurrence of species is a product of chance, historical patterns of speciation and migration, dispersal, abiotic environmental factors, and biotic interactions, with none of these processes being mutually exclusive. Here we present a survey and meta‐analyses of 59 papers that compare observed patterns in plant communities with null models simulating random patterns of species assembly. According to the type of data under study and the different methods that are applied to detect community assembly, we distinguish four main types of approach in the published literature: species co‐occurrence, niche limitation, guild proportionality and limiting similarity. Results from our meta‐analyses suggest that non‐random co‐occurrence of plant species is not a widespread phenomenon. However, whether this finding reflects the individualistic nature of plant communities or is caused by methodological shortcomings associated with the studies considered cannot be discerned from the available metadata. We advocate that more thorough surveys be conducted using a set of standardized methods to test for the existence of assembly rules in data sets spanning larger biological and geographical scales than have been considered until now. We underpin this general advice with guidelines that should be considered in future assembly rules research. This will enable us to draw more accurate and general conclusions about the non‐random aspect of assembly in plant communities.     

UNCOUPLING OF SILICON COMPARED WITH CARBON AND NITROGEN METABOLISMS AND THE ROLE OF THE CELL CYCLE IN CONTINUOUS CULTURES OF THALASSIOSIRA PSEUDONANA (BACILLARIOPHYCEAE) UNDER LIGHT,NITROGEN, AND PHOSPHORUS CONTROL1

The elemental composition and the cell cycle stages of the marine diatom Thalassiosira pseudonana Hasle and Heimdal were studied in continuous cultures over a range of different light‐ (E), nitrogen‐ (N), and phosphorus‐ (P) limited growth rates. In all growth conditions investigated, the decrease in the growth rate was linked with a higher relative contribution of the G2+M phase. The other phases of the cell cycle, G1 and S, showed different patterns, depending on the type of limitation. All experiments showed a highly significant increase in the amount of biogenic silica per cell and per cell surface with decreasing growth rates. At low growth rates, the G2+M elongation allowed an increase of the silicification of the cells. This pattern could be explained by the major uptake of silicon during the G2+M phase and by the independence of this process on the requirements of the other elements. This was illustrated by the elemental ratios Si/C and Si/N that increased from 2‐ to 6‐fold, depending of the type of limitation, whereas the C/N ratio decreased by 10% (E limitation) or increased by 50% (P limitation). The variations of the ratios clearly demonstrate the uncoupling of the Si metabolism compared with the C and N metabolisms. This uncoupling enabled us to explain that in any of the growth condition investigated, the silicification of the cells increased at low growth rates, whereas carbon and nitrogen cellular content are differently regulated, depending of the growth conditions.     

Management oriented forest map of South India: thematic derivations

The principles and methods of the vegetation mapping undertaken at the French Institute, Pondicherry, are dealt with herein. Particularly, the characterisation of the different types of vegetation and especially the originality of the method: the dynamic interpretation of the vegetation and the depiction of the bioclimatic conditions. The programme of the forest map of South India at scale 1:250 000, undertaken in collaboration with the forest departments of the concerned states, is then described with special attention given to the source and the collection of data. This map has been conceived to serve as a basic document for the sustainable management of the forests. Three examples of its application are given. They concern the detection of anomalies between the existing forest cover and the prevalent climatic environment; the detection of areas for which protection is urgently needed; the selection of regions showing a particular interest in the field of nature conservation or as gene pool reserve. Finally, an example of a thematic map of sensibility of the forests is given, using the vegetation map as a basis.     

Subsets of sialylated, sulfated mucins of diverse origins are recognized by L-selectin. Lack of evidence for unique oligosaccharide sequences mediating binding

Previous studies have shown that the mucin-type polypeptidesGlyCAM-1, CD34, and MAdCAM-1 can function as ligands for L-selectinonly when they are synthesized by the specialized high-endothelialvenules (HEV) of lymph nodes. Since sialylation, sulfation,and possibly fucosylation are required for generating recognition,we reasoned that other mucins known to have such componentsmight also bind L-selectin. We show here that soluble mucinssecreted by human colon carcinoma cells, as well as those derivedfrom human bronchial mucus can bind to human L-selectin in acalcium-dependent manner. As with GlyCAM-1 synthesized by lymphnode HEY, 2–3 linked sialic acids and sulfation seem toplay a critical role in generating this L-selectin binding.In each case, only a subset of the mucin molecules is recognizedby L-selectin. Binding is not destroyed by boiling, suggestingthat recognition may be based primarily upon carbohydrate structures.Despite this, O-linked oligosaccharide chains released fromthese ligands by beta-elimination do not show any detectablebinding to L-selectin. Following protease treatment of the ligands,binding persists in a subset of the resulting fragments, indicatingthat specific recognition is determined by certain regions ofthe original mucins. How ever, O-linked oligosaccharides releasedfrom the subset of non-binding mucin fragments do not show verydifferent size and charge profiles compared to those that dobind. Furthermore, studies with polylactosamine-degrading endoglycosidasessuggest that the core structures involved in generating bindingcan vary among the different ligands. Taken together, thesedata indicate that a single unique oligosaccharide structuremay not be responsible for high-affinity binding. Rather, diversemucins with sialylated, sulfated, fucosylated lactosamine-typeO-linked oligosaccharides can generate high-affinity L-selectinligands, but only when they present these chains in unique spacingand/or clustered combinations, presumably dictated by the polypeptidebackbone. L-selectin mucins sialic sialic acid sulfate adhesion     

Alternate substrates and inhibitors of bacterial 4-hydroxyphenylpyruvate dioxygenase

A variety of analogues of (4-hydroxyphenyl)pyruvic acid were synthesized, and the reactions of these compounds with the 4-hydroxyphenylpyruvate dioxygenase from Pseudomonas sp. P.J. 874 were examined. Several of the ring-substituted substrate analogues are reversible inhibitors of the enzyme, the most potent being the competitive inhibitor (2,6-difluoro-4-hydroxyphenyl) pyruvate (Ki = 1.3 microM). Two substrate analogues (2-fluoro-4-hydroxyphenyl)pyruvate and [(4-hydroxyphenyl)thio]pyruvate proved to be alternate substrates for the enzyme. The former compound is converted to (3-fluoro-2,5-dihydroxyphenyl)acetate in an essentially normal catalytic sequence including oxidative decarboxylation, ring hydroxylation, and side-chain migration. The latter compound, however, undergoes oxidative decarboxylation and sulfoxidation to give [(4-hydroxyphenyl)sulfinyl]acetate; ring oxidation is not observed. The implications of these results with regard to the catalytic mechanism of 4-hydroxyphenylpyruvate dioxygenase are discussed.     

Mapping of a Gene for Long QT Syndrome to Chromosome 4q25-27

Long QT syndrome (LQTS) is a heterogeneous inherited disorder causing syncope and sudden death from ventricular arrhythmias. A first locus for this disorder was mapped to chromosome 11p15.5. However, locus heterogeneity has been demonstrated in several families, and two other loci have recently been located on chromosomes 7q35-36 and 3p21-24. We used linkage analysis to map the locus in a 65-member family in which LQTS was associated with more marked sinus bradycardia than usual, leading to sinus node dysfunction. Linkage to chromosome 11p15.5, 7q35-36, or 3p21-24 was excluded. Positive linkage was obtained for markers located on chromosome 4q25-27. A maximal LOD score of 7.05 was found for marker D4S402. The identification of a fourth locus for LQTS confirms its genetic heterogeneity. Locus 4q25-27 is associated with a peculiar phenotype within the LQTS entity.     

Spatial, temporal, and species variation in prevalence of influenza A viruses in wild migratory birds

Although extensive data exist on avian influenza in wild birds in North America, limited information is available from elsewhere, including Europe. Here, molecular diagnostic tools were employed for high-throughput surveillance of migratory birds, as an alternative to classical labor-intensive methods of virus isolation in eggs. This study included 36,809 samples from 323 bird species belonging to 18 orders, of which only 25 species of three orders were positive for influenza A virus. Information on species, locations, and timing is provided for all samples tested. Seven previously unknown host species for avian influenza virus were identified: barnacle goose, bean goose, brent goose, pink-footed goose, bewick's swan, common gull, and guillemot. Dabbling ducks were more frequently infected than other ducks and Anseriformes; this distinction was probably related to bird behavior rather than population sizes. Waders did not appear to play a role in the epidemiology of avian influenza in Europe, in contrast to the Americas. The high virus prevalence in ducks in Europe in spring as compared with North America could explain the differences in virus-host ecology between these continents. Most influenza A virus subtypes were detected in ducks, but H13 and H16 subtypes were detected primarily in gulls. Viruses of subtype H6 were more promiscuous in host range than other subtypes. Temporal and spatial variation in influenza virus prevalence in wild birds was observed, with influenza A virus prevalence varying by sampling location; this is probably related to migration patterns from northeast to southwest and a higher prevalence farther north along the flyways. We discuss the ecology and epidemiology of avian influenza A virus in wild birds in relation to host ecology and compare our results with published studies. These data are useful for designing new surveillance programs and are particularly relevant due to increased interest in avian influenza in wild birds.     

No evidence for a tumor necrosis factor α stimulated 2-methylaminoisobutyric acid uptake in hepatocyte monolayer

This study investigates the short-tem effects of glucagon and human recombinant tumor necrosis factor α (TNFα) singly and in association on 2-methylaminoisobutyric acid (MeAIB) transport in hepatocyte monolayers. As expected, glucagon induced a time-dependent stimulation of MeAIB transport. In our experimental conditions, TNFα did not induce cytolysis. A 2 hour exposure to TNFα (0.05–500 ng/I) with or without glucagon (10^?9 to 10^?6 M) did not modify the basal or glucagon-stimulated MeAIB transport. Varying the duration of exposure to TNFα 5 ng/I up to 6 h was equally ineffective. The presence of hydrocortisone potentiated the glucagon-stimulated transport, but TNFα remained ineffective. Finally, the association of interferon (IFNγ) with TNFα and/or glucagon was unable to modify the transport activity. These data demonstrate that TNFα does not exert a direct effect on MeAIB transport in hepatocytes, at least on a short-term basis. © 1995 Wiley-Liss, Inc.     

Habitat utilization by sympatric European mink<Emphasis Type="Italic">Mustela lutreola</Emphasis> and polecats<Emphasis Type="Italic">Mustela putorius</Emphasis> in south-western France

The European minkMustela lutreola Linnaeus, 1761 and the European polecatMustela putorius Linnaeus, 1758 are related species sympatric in southwestern France. The European mink is rapidly disappearing whereas the polecat maintains good populations. Seasonal habitat use of both species was compared in the Landes de Gascogne region to identify if some vulnerability factors of the European mink were associated with habitats occupied by this mustelid. Potential habitats were mapped using a satellite picture and 12 main types of habitats were defined. Animal locations were recorded by radiotracking 9 European mink and 14 polecats from March 1996 to August 1999. Resting animals were located by triangulation, and, when possible, resting places were described. Animals in activity were tracked by continuous monitoring. Data collected revealed a strong preference of European mink for flooded habitats, particularly open marshes, flooded woodlands and moorlands. They seldom left the corridor of the riparian forest and their resting places were mainly in flooded environments, above ground (under herbs or bushes) or in cavities between tree roots. European polecats were less tightly linked to wetlands. Most of their locations were in the pine forests outside the valleys and their resting places were mainly in burrows. The strong specialisation of European mink in aquatic habitats is probably one of the main reasons for its decline because wetlands suffer drastic damages throughout all of its range. Maintaining adequate water levels is crucial for satisfying habitat requirements of mink.